One popular Erik and Christine baby fanfiction might be 'The New Life of the Phantom's Family'. It beautifully depicts their journey as new parents, filled with love and challenges.
In a different version, Christine gets lost in the opera house and Erik is the one who finds her. He could have been watching her from afar all this time. Instead of scaring her, he gently leads her to a safe place. They start talking about their dreams and fears. Christine realizes that Erik is not the monster everyone makes him out to be. This new understanding blossoms into love as they continue to meet secretly and share their innermost thoughts.
A simple fanfiction could be that Erik writes a beautiful song for Christine. When she hears it, she is deeply moved. She seeks him out to thank him. This encounter starts a series of meetings where they slowly fall in love. Their love is based on mutual respect for each other's talents.
There's also the theme of sacrifice. Sometimes Erik sacrifices his own desires for Christine's happiness, and vice versa. It adds a layer of depth to their relationship. And the exploration of their different social standings, with Erik being an outcast and Christine being a more 'normal' part of society, is another common theme in these fanfictions.
A unique way to develop their relationship could be through a shared dream or vision. They both see a future together that is full of love and acceptance. This shared experience makes them more determined to be together. Christine starts to see past Erik's physical appearance and focuses on the beauty of his soul. They then start to explore their love in a more physical way, like holding hands and then gradually becoming more affectionate.
Love and passion are very common themes. Since lemon often implies a certain level of romantic or sexual tension, these fanfictions usually explore the deep love between Erik and Christine in a more intense way.
One possible plot could be that Erik, being the complex and often misunderstood character, kidnaps Christine out of his obsessive love for her. He might take her to his lair, thinking he can protect her from the outside world which he believes is full of people who don't truly appreciate her talent. Then Christine could struggle between her fear of Erik and her understanding of his tortured soul.